The present invention relates to the field of gas turbine engines and deals more specifically with the operations involved in mounting the engine shaft.
A twin-spool turbojet engine with a front-mounted fan, for example, comprises a low-pressure spool, known as the LP spool, and a high-pressure spool known as the HP spool.
By convention, in the present application, the terms “upstream” and “downstream” are defined with respect to the direction in which air flows through the turbojet engine. Thus, a twin-spool turbojet engine with a front-mounted fan comprises, in the conventional way, from the upstream end downstream, a fan, an LP compressor stage, an HP compressor stage, a combustion chamber, an HP turbine stage and an LP turbine stage.
By convention, in this application, the terms “interior” and “exterior” are defined radially with respect to the axis of the engine. Thus, a cylinder extending along the axis of the engine comprises an interior face facing toward the axis of the engine and an exterior surface, on the opposite side to its interior surface.
The shaft of the LP spool is guided in its rotation in bearings supported by the fixed structure of the engine whereas the shaft of the HP spool is guided by bearings supported by the LP spool, the shafts of the two spools being concentric.
The HP turbine disk is secured to a journal for the mounting of the bearing that allows the HP shaft to rotate with respect to the LP shaft, this bearing being known as bearing “4” in a twin-spool engine with a front-mounted fan.
The LP shaft is housed in a cylindrical sleeve secured to the journal. When the sleeve is mounted in the engine, it is necessary to fix the position of the sleeve with respect to the journal, both axially and tangentially.
To do this, with reference to FIG. 1, the journal 1 comprises an axial cylindrical part 13 on which dog couplings are formed. The dog couplings are in the form of a crenellated radial annular band 20 comprising projecting teeth 21 alternating with crenels 22. Hereinafter, a crenel is defined as being the space between two successive teeth.
Because the dog coupling teeth 21 of the journal 1 are directed inward, that is to say toward the axis of the engine, the dog couplings of the journal will be termed interior dog couplings.
The interior dog couplings of the journal are designed to be in register with dog couplings, known as exterior dog couplings, formed on an annular band of the sleeve, the teeth of the exterior dog couplings being directed outward.
Axial alignment between the teeth of the interior and exterior dog couplings allows the sleeve to be immobilized axially with respect to the journal, the upstream face of the teeth of the exterior dog couplings of the sleeve being in axial abutment against the downstream face of the teeth of the interior dog couplings of the journal. The sleeve is tangentially immobilized by upstream-directed axial engagement teeth formed on the labyrinth ring of the bearing which is mounted downstream of the sleeve. The axial engagement teeth are introduced between the teeth of the interior and exterior dog couplings, thus preventing the sleeve from rotating with respect to the journal about the axis of the engine. A locknut is screwed axially, in the cold state, onto the downstream end of the journal, to keep the sleeve axially and tangentially immobilized.
The axial clamping applied by the locknut to the sleeve leads to high axial stresses on the interior dog couplings of the journal. The sharp edges and short radii of the teeth of the interior dog couplings lead to fatigue in the journal near said edges. The build-up of stresses of various kinds in the upstream part of the journal weakens the journal.
The journal further comprises lubricating ducts placing the oil stored outside the journal in communication with the lubrication space in which the bearing supported by the journal is bathed. These internal lubricating ducts are formed longitudinally in the journal using an EDM (electron-discharge-machining) method. A method such as this has numerous disadvantages. First of all, it leads to an internal mechanical weakening of the machined zone. The thermally affected zone loses its mechanical properties, leading to a potential risk for damage. Further, the internal ducts formed by EDM are difficult to inspect because of their size and location on the component. After they have been formed, they may have internal cracks liable to weaken the journal.
Finally, the internal lubricating ducts open into the lubricating space via radial oil removal ports. The sharp edges of the ports lead to mechanical weaknesses in the journal.
Thus, the journal is the site of a buildup of mechanical fatigue phenomena which are liable to weaken the journal as it operates.